Semiconductor device using shallow trench isolation and method of fabricating the same

ABSTRACT

A semiconductor device adopting shallow trench isolation for reducing an internal stress of a semiconductor substrate. The semiconductor device is composed of a semiconductor substrate provided with a trench for isolation, and an insulating film formed to cover the trench for relaxing an internal stress of the semiconductor substrate. The insulating film includes a first portion disposed to be opposed to a bottom of the trench, and a second portion disposed to be opposed to a side of the trench. A first thickness of the first portion is different from a second thickness of the second portion.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a semiconductor device and amethod of fabricating the same. More particularly, the present inventionrelates to a semiconductor device using shallow trench isolation.

[0003] 2. Description of the Related Art

[0004] Highly integrated semiconductor devices often adopt shallowtrench isolation. Isolation has been conventionally achieved by a LOCOS(Local Oxidation of Silicon) technique. However, the LOCOS technique hasdifficulties in fabricating highly integrated semiconductor devices,such as bird's beaks and poor isolation abilities. This situationencourages the adoption of the shallow trench isolation.

[0005]FIG. 1 shows a conventional structure of a semiconductor devicesusing shallow trench isolation. A trench 102 is formed on a siliconsubstrate 101. The inner surface of the trench 102 is coated by asilicon oxide film 103 formed by thermal oxidization. A liner 104 madeof silicon nitride is formed on the silicon oxide film 103. A siliconoxide film 105 is formed on the liner 104 to fill the trench 102.

[0006] The liner 104 relaxes the interior stress induced in the siliconsubstrate 101. The shallow trench isolation causes the interior stressto be applied to the silicon substrate 161, because of the difference inthermal expansion coefficients between the silicon substrate 101 and thesilicon oxide film 105. The stress applied to the silicon substrate 101induces crystal defects in the silicon substrate, and thus increasesjunction leak currents. The increase in the junction leak currentscauses improper operations of semiconductor devices. The relaxation ofthe internal stress by the liner 104 reduces the junction leak currentand thus improves the reliability of the semiconductor device.

[0007] It is desired that the internal stress of the semiconductorsubstrate be further reduced. The reduction of the internal stressefficiently improves the reliability of the semiconductor device.

SUMMARY OF THE INVENTION

[0008] Therefore, an object of the present invention is to provide atechnique for further reducing the internal stress applied to thesemiconductor substrate of the semiconductor device in which the shallowtrench isolation is employed.

[0009] In an aspect of the present invention, a semiconductor deviceincludes a semiconductor substrate provided with a trench for isolation,and an insulating film formed to cover the trench for relaxing aninternal stress of the semiconductor substrate. The insulating filmincludes a first portion opposed to a bottom of the trench, and a secondportion opposed to a side of the trench. A first thickness of the firstportion is different from a second thickness of the second portion.

[0010] The first thickness of the first portion may be thinner than thesecond thickness of the second portion.

[0011] When the semiconductor device further includes another insulatingfilm in the trench and the other insulating film exerts a compressivestress on the semiconductor substrate, the insulating film preferablyexerts a tensile stress on said semiconductor substrate.

[0012] The insulating film is preferably formed of one selected from agroup consisting of silicon nitride and silicon oxinitride.

[0013] In another aspect of the present invention, a semiconductordevice includes a semiconductor substrate provided with a trench forisolation, and an insulating film formed to cover the trench forrelaxing an internal stress of the semiconductor substrate. Theinsulating film is opposed to a side of the trench and is not opposed toa bottom of the trench.

[0014] In still another aspect of the present invention, a semiconductordevice includes a semiconductor substrate provided with a trench forisolation, a silicon oxide film formed to cover the trench, and aninsulating film formed on the silicon oxide film. The insulating filmexerts a tensile stress on the semiconductor substrate. The insulatingfilm includes a first portion opposed to a bottom of the trench, and asecond portion opposed to a side of the trench. A first thickness of thefirst portion is thinner than a second thickness of the second portion.

[0015] In still another aspect of the present invention, a semiconductordevice includes a semiconductor substrate provided with a trench forisolation, a silicon oxide film formed to cover the trench, and aninsulating film formed on the silicon oxide film. The insulating filmexerts a tensile stress on the semiconductor substrate. The insulatingfilm is opposed to a side of the trench and is not opposed to a bottomof the trench.

[0016] In still another aspect of the present invention, a semiconductordevice includes a semiconductor substrate provided with a trench forisolation, a silicon oxide film formed to cover the trench, and aninsulating film disposed on the silicon oxide film. The insulating filmis formed of one selected from a group consisting of silicon nitride andsilicon oxinitride. The insulating film includes a first portion opposedto a bottom of the trench, and a second portion opposed to a side of thetrench. A first thickness of the first portion is thinner than a secondthickness of the second portion.

[0017] In still another aspect of the present invention, a semiconductordevice includes a semiconductor substrate provided with a trench forisolation, a silicon oxide film formed to cover the trench, and aninsulating film disposed on the silicon oxide film. The insulating filmis formed of one selected from a group consisting of silicon nitride andsilicon oxinitride. The insulating film is opposed to a side of thetrench and is not opposed to a bottom of the trench.

[0018] In still another aspect of the present invention, a method offabricating a semiconductor device is composed of:

[0019] forming a trench for isolation in the semiconductor substrate;

[0020] forming an insulating film to cover the trench for relaxing aninternal stress of the silicon substrate. The insulating film includes afirst portion opposed to a bottom of the trench, and a second portionopposed to a side of the trench. The first thickness of the firstportion is different from a second thickness of the second portion. Thefirst thickness of the first portion may be thinner than the secondthickness of the second portion.

[0021] In still another aspect of the present invention, a method forfabricating a semiconductor device is composed of:

[0022] forming a trench for isolation in a semiconductor substrate;

[0023] forming an insulating film to cover the trench for relaxing aninternal stress of the silicon substrate, wherein the insulating film isopposed to a side of the trench and is not opposed to a bottom of thetrench.

[0024] In still another aspect of the present invention, a method forfabricating a semiconductor device is composed of:

[0025] forming a trench for isolation in a semiconductor substrate;

[0026] forming a silicon oxide film to cover the trench; and

[0027] forming an insulating film on the silicon oxide film, wherein theinsulating film exerts a compressive stress on the silicon substrate.The insulating film includes a first portion opposed to a bottom of thetrench, and a second portion opposed to a side of the trench. A firstthickness of the first portion is thinner than a second thickness of thesecond portion.

[0028] In still another aspect of the present invention, a method forfabricating a semiconductor device is composed of:

[0029] forming a trench for isolation in a semiconductor substrate;

[0030] forming a silicon oxide film to cover the trench;

[0031] forming an insulating film on the silicon oxide film. Theinsulating film is opposed to a side of the trench and is not opposed toa bottom of the trench.

[0032] In still another aspect of the present invention, a method forfabricating a semiconductor device is composed of:

[0033] forming a trench for isolation in a semiconductor substrate;

[0034] forming a silicon oxide film to cover the trench; and

[0035] forming an insulating film on the silicon oxide film. Theinsulating film is formed of one selected from a group consisting ofsilicon nitride and silicon oxinitride. The insulating film includes afirst portion opposed to a bottom of the trench, and a second portionopposed to a side of the trench. A first thickness of the first portionis different from a second thickness of the second portion.

[0036] In still another aspect of the present invention, a method forfabricating a semiconductor device is composed of:

[0037] forming a trench for isolation in a semiconductor substrate;

[0038] forming a silicon oxide film to cover the trench; and

[0039] forming an insulating film on the silicon oxide film. Theinsulating film is formed of one selected from a group consisting ofsilicon nitride and silicon oxinitride. The insulating film is opposedto a side of the trench and is not opposed to a bottom of the trench.

BRIEF DESCRIPTION OF THE DRAWINGS

[0040]FIG. 1 shows a conventional semiconductor device adopting shallowtrench isolation;

[0041]FIG. 2 shows a semiconductor device in an embodiment of thepresent invention;

[0042]FIG. 3 shows internal stresses exerted on a silicon substrate;

[0043]FIG. 4 shows a modification of the semiconductor device in theembodiment; and

[0044] FIGS. 5 to 12 are section views showing a fabrication process ofthe semiconductor device in the embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0045] An embodiment of a semiconductor apparatus according to thepresent invention and a method of manufacturing the same will bedescribed below with reference to the attached drawings.

[0046] In an embodiment of the semiconductor device according to thepresent invention, as shown in FIG. 2, a trench 2 is formed on a siliconsubstrate 1. An inner surface of the trench 2 is coated by a siliconoxide film 3. The silicon oxide film 3 is coated by a liner 4. A siliconoxide film 5 is formed on the liner 4. The trench 2 is filled with thesilicon oxide film 3, the liner 4 and the silicon oxide film 5.

[0047] The liner 4 is formed of an insulating film that has acompressive internal stress. The silicon oxide films 3, 5 have thetensile internal stresses, and thus exerts compression stresses on thesilicon substrate 1. The liner 4, which has a compressive internalstress, exerts a tensile stress on the silicon substrate 1, and thusrelaxes the internal stress of the silicon substrate 1. The liner 4 ispreferably formed of silicon nitride or silicon oxinitride.

[0048] The liner 4 includes a bottom portion that is opposed to thebottom surface 2 a of the trench 2, and a side portion that is opposedto the side surface 2 b of the trench 2. A film thickness X of thebottom portion of the liner 4 is thinner than a film thickness Y of theside portion of the liner 4. This structure further reduces the internalstress of the silicon substrate 1. The inventor has discovered that aninternal stress of the silicon substrate 1 in the vicinity of the bottom2 a of the trench 2 is weaker than that in the vicinity of the side 2 bof the trench 2. The difference in the film thicknesses of the bottomand side portions causes the different stresses on the vicinity of thebottom surface 2 a and side surface 2 b, and effectively reduces theinternal stress of the silicon substrate 1.

[0049]FIG. 3 shows the internal stresses of the silicon substrate 1 atpositions A and B indicated by the circles in FIG. 2. The position A islocated near the bottom surface 2 a of the trench 2, and the position Bis located near the side of the trench 2. The internal stresses of thesilicon substrate 1 are measured by a convergent beam electrondiffraction (CBED) method.

[0050] Three samples have been fabricated; a first sample has astructure in which the liner 4 is excluded from the structure shown inFIG. 2, a second sample has a structure in which the liner 4 isconformal or uniform in thickness, and a third sample has the structureshown in FIG. 2. A line 11 in FIG. 3 represents the internal stresses ofthe silicon substrate without the liner 4. A line 12 represents theinternal stresses of the silicon substrate 1 when the liner 4 is uniformin the film thickness. A line 13 represents the internal stresses of thesilicon substrate 1 in the semiconductor device having the structureshown in FIG. 2.

[0051] In regard to the case that the film thickness of the liner 4 isuniform, the internal stress of the silicon substrate 1 is weak at theposition B as indicated by the line 12. However, the considerably strongtensile stress is applied to the silicon substrate 1 at the position A,which is located near the bottom plane of the trench 2. This impliesthat the uniform liner 4 exerts the excessive tensile stress on thesilicon substrate 1 at the position A.

[0052] The line 13 implies that the structure in which the filmthickness X of the bottom portion of the liner 4 is thinner than thefilm thickness Y of the side portion remarkably reduces the internalstress of the silicon substrate 1 at the position A. The thinner filmthickness of the bottom portion prevents an excessive tensile stressfrom being exerted on the silicon substrate 1.

[0053] With reference to FIG. 2, the film thickness X of the bottomportion of the liner 4 is adjusted on the basis of the internal stressapplied to the silicon substrate 1. It should be noted that the adjustedfilm thickness X of the bottom portion might be zero as shown in FIG. 4.That is, the liner 4 may be designed such that the liner 4 is notopposed to the bottom surface 2 a of the trench 2.

[0054] The method of fabricating the semiconductor device shown in FIG.2 will be described below.

[0055] As shown in FIG. 5, the surface of the silicon substrate 1 isthermally oxidized to thereby form a sacrificed oxide film 6. A filmthickness of the sacrificed oxide film 6 is typically 10 to 20 nm.

[0056] A silicon nitride film 7 is then formed on the sacrificed oxidefilm 6, as shown in FIG. 6. A film thickness of the silicon nitride film7 is typically 100 to 200 nm.

[0057] After the formation of the silicon nitride film 7, as shown inFIG. 7, the sacrificed oxide film 6 and the silicon nitride film 7 arepatterned by an etching technique, which is well known in the art. Thesilicon substrate 1 is then etched to form the trench 2 by using thepatterned sacrificed oxide film 6 and silicon nitride film 7 as a mask.

[0058] After the formation of the trench 2, the inner surface of thetrench 2 is thermally oxidized to thereby form the silicon oxide film 3.A film thickness of the silicon oxide film 3 is typically 10 to 30 nm.

[0059] A silicon nitride film 8 is then formed to cover the entirestructure as shown in FIG. 8. The silicon nitride film 8 is exemplarilyformed by using an LPCVD (Low Pressure Chemical Vapor Deposition)method. A film thickness of the silicon nitride film 8 is substantiallyuniform, and is typically 3 to 10 nm. The silicon nitride film 8 isformed in the conditions that the formed silicon nitride film 8 has thecompressive internal stress. The other insulation film having acompressive internal stress may be used instead of the silicon nitridefilm 8. For example, a silicon oxide nitride film may be formed insteadof the silicon nitride film 8.

[0060] After the formation of the silicon nitride film 8, as shown inFIG. 9, the silicon nitride film 8 is anisotropically etched in thedirection perpendicular to the major surface of the silicon substrate 1.A bottom portion of the silicon nitride film 8, which is opposed to thebottom surface 2 a, is partially removed. This causes the film thicknessof the bottom portion of the silicon nitride film 8 to be thinner thanthat of the side portion opposed to the side 2 b of the trench 2. Itshould be noted that the bottom portion of the silicon nitride film 8might be totally removed to form the structure shown in FIG. 4. Theetching of the silicon nitride film 8 may be executed by the physicaletching such as ion milling or the physical and chemical etching.

[0061] A silicon oxide film 9 is then formed to cover the entirestructure by using a CVD method as shown in FIG. 10. The trench 2 istotally filled with the silicon oxide film 9.

[0062] After the formation of the silicon oxide film 9, as shown in FIG.11, a portion of the silicon oxide film 9 outside the trench 2 isremoved to expose the surface of the silicon nitride film 8. The partialremoval of the silicon oxide film 9 is achieved by a CMP (ChemicalMechanical Polishing) method. The silicon nitride film 8 and the siliconnitride film 7 function as stoppers during the CMP process.

[0063] The silicon substrate 1 is then processed by heated phosphoricacid to remove the silicon nitride film 7 and a portion of the siliconnitride film 8 outside the trench 2 as shown in FIG. 12. The remainingportion of the silicon nitride film 8 constitutes the liner 4.

[0064] The silicon substrate 1 is then processed by hydrofluoric acid toremove the sacrificed oxide film 6 and a portion of the silicon oxidefilm 9 outside the trench 2. The removal of the sacrificed oxide film 6and the portion of the silicon oxide film 9 completes the formation ofthe structure shown in FIG. 2 (or, FIG. 4). MOS transistors and otherelements are then formed on the silicon substrate 1 to fabricate an LSI(Large Scale Integrated circuit).

[0065] As described, the semiconductor device in this embodiment has thestructure in which the film thickness X of the bottom portion of theliner 4 is thinner than the film thickness Y of the side portion, or thefilm thickness X of the bottom portion is zero. This structureeffectively reduces the internal stress of the silicon substrate 1.

[0066] Although the invention has been described in its preferred formwith a certain degree of particularity, it is understood that thepresent disclosure of the preferred form has been changed in the detailsof construction and the combination and arrangement of parts may beresorted to without departing from the spirit and the scope of theinvention as hereinafter claimed.

What is claimed is:
 1. A semiconductor device comprising: asemiconductor substrate provided with a trench for isolation; and aninsulating film formed to cover said trench for relaxing an internalstress of said semiconductor substrate, wherein said insulating filmincludes: a first portion disposed to be opposed to a bottom of saidtrench, and a second portion disposed to be opposed to a side of saidtrench, and wherein a first thickness of said first portion is differentfrom a second thickness of said second portion.
 2. The semiconductordevice according to claim 1, wherein said first thickness of said firstportion is thinner than said second thickness of said second portion. 3.The semiconductor device according to claim 2, further comprisinganother insulating film in said trench, wherein said another insulatingfilm exerts a compressive stress on said semiconductor substrate, andwherein said insulating film exerts a tensile stress on saidsemiconductor substrate.
 4. The semiconductor device according to claim2, wherein said insulating film is formed of one selected from a groupconsisting of silicon nitride and silicon oxinitride.
 5. A semiconductordevice comprising: a semiconductor substrate provided with a trench forisolation; and an insulating film formed to cover said trench forrelaxing an internal stress of said semiconductor substrate, whereinsaid insulating film is opposed to a side of said trench and is notopposed to a bottom of said trench.
 6. The semiconductor deviceaccording to claim 5, further comprising another insulating film in saidtrench, wherein said another insulating film exerts a compressive stresson said semiconductor substrate, and wherein said insulating film exertsa tensile stress on said semiconductor substrate.
 7. The semiconductordevice according to claim 5, wherein said insulating film is formed ofone selected from a group consisting of silicon nitride and siliconoxinitride.
 8. A semiconductor device comprising: a semiconductorsubstrate provided with a trench for isolation; a silicon oxide filmformed to cover said trench; and an insulating film formed on saidsilicon oxide film, wherein said insulating film exerts a tensile stresson said semiconductor substrate, and wherein said insulating filmincludes: a first portion disposed to be opposed to a bottom of saidtrench, and a second portion disposed to be opposed to a side of saidtrench, and wherein a first thickness of said first portion is thinnerthan a second thickness of said second portion.
 9. The semiconductordevice according to claim 8, wherein said insulating film is formed ofone selected from a group consisting of silicon nitride and siliconoxinitride.
 10. A semiconductor device comprising: a semiconductorsubstrate provided with a trench for isolation; a silicon oxide filmformed to cover said trench; and an insulating film formed on saidsilicon oxide film, wherein said insulating film exerts a tensile stresson said semiconductor substrate, and, wherein said insulating film isopposed to a side of said trench and is not opposed to a bottom of saidtrench.
 11. The semiconductor device according to claim 10, wherein saidinsulating film is formed of one selected from a group consisting ofsilicon nitride and silicon oxinitride.
 12. A semiconductor devicecomprising: a semiconductor substrate provided with a trench forisolation; a silicon oxide film formed to cover said trench; and aninsulating film disposed on said silicon oxide film, wherein saidinsulating film is formed of one selected from a group consisting ofsilicon nitride and silicon oxinitride, and wherein said insulating filmincludes: a first portion disposed to be opposed to a bottom of saidtrench, and a second portion disposed to be opposed to a side of saidtrench, and wherein a first thickness of said first portion is thinnerthan a second thickness of said second portion.
 13. A semiconductordevice comprising: a semiconductor substrate provided with a trench forisolation; a silicon oxide film formed to cover said trench; and aninsulating film disposed on said silicon oxide film, wherein saidinsulating film is formed of one selected from a group consisting ofsilicon nitride and silicon oxinitride, and wherein said insulating filmis opposed to a side of said trench and is not opposed to a bottom ofsaid trench.
 14. A method of fabricating a semiconductor devicecomprising: forming a trench for isolation in said semiconductorsubstrate; and forming an insulating film to cover said trench forrelaxing an internal stress of said silicon substrate, wherein saidinsulating film includes: a first portion disposed to be opposed to abottom of said trench, and a second portion disposed to be opposed to aside of said trench, and wherein a first thickness of said first portionis different from a second thickness of said second portion.
 15. Themethod according to claim 14, wherein said first thickness of said firstportion is thinner than said second thickness of said second portion.16. The method according to claim 15, further comprising: forminganother insulating film in said trench, wherein said another insulatingfilm exerts a compressive stress on said semiconductor substrate, andsaid insulating film exerts a tensile stress on said semiconductorsubstrate.
 17. The method according to claim 15, wherein said insulatingfilm is formed of one selected from a group consisting of silicon oxideand silicon oxinitride.
 18. A method for fabricating a semiconductordevice comprising: forming a trench for isolation in a semiconductorsubstrate; and forming an insulating film to cover said trench forrelaxing an internal stress of said silicon substrate, wherein saidinsulating film is opposed to a side of said trench and is not opposedto a bottom of said trench.
 19. The method according to claim 18,further comprising: forming another insulating film in said trench,wherein said another insulating film exerts a compressive stress on saidsemiconductor substrate, and said insulating film exerts a tensilestress on said semiconductor substrate.
 20. The method according toclaim 18, wherein said insulating film is formed of one selected from agroup consisting of silicon oxide and silicon oxinitride.
 21. A methodfor fabricating a semiconductor device comprising: forming a trench forisolation in a semiconductor substrate; forming a silicon oxide film tocover said trench; and forming an insulating film on said silicon oxidefilm, wherein said insulating film exerts a compressive stress on saidsilicon substrate, and wherein said insulating film includes: a firstportion disposed to be opposed to a bottom of said trench, and a secondportion disposed to be opposed to a side of said trench, and wherein afirst thickness of said first portion is thinner than a second thicknessof said second portion.
 22. The method according to claim 21, whereinsaid insulating film is formed of one selected from a group consistingof silicon oxide and silicon oxinitride.
 23. A method for fabricating asemiconductor device comprising: forming a trench for isolation in asemiconductor substrate; forming a silicon oxide film to cover saidtrench; and forming an insulating film on said silicon oxide film,wherein said insulating film is opposed to a side of said trench and isnot opposed to a bottom of said trench.
 24. The method according toclaim 23, wherein said insulating film is formed of one selected from agroup consisting of silicon oxide and silicon oxinitride.
 25. A methodfor fabricating a semiconductor device comprising: forming a trench forisolation in a semiconductor substrate; forming a silicon oxide film tocover said trench; and forming an insulating film on said silicon oxidefilm, wherein said insulating film is formed of one selected from agroup consisting of silicon nitride and silicon oxinitride, wherein saidinsulating film includes: a first portion disposed to be opposed to abottom of said trench, and a second portion disposed to be opposed to aside of said trench, and wherein a first thickness of said first portionis different from a second thickness of said second portion.
 26. Amethod for fabricating a semiconductor device comprising: forming atrench for isolation in a semiconductor substrate; forming a siliconoxide film to cover said trench; and forming an insulating film on saidsilicon oxide film, wherein said insulating film is formed of oneselected from a group consisting of silicon nitride and siliconoxinitride, wherein said insulating film is opposed to a side of saidtrench and is not opposed to a bottom of said trench.